An inductor, also called a coil or reactor, is a passive two-terminal electrical component that resists changes in electric current passing through it. Inductors are used in analog circuits and signal processing benefiting applications in power supply, electric utility, and radio frequency. In the manufacture of electronic equipment, tunable inductors are often utilized. Due to its miniaturization, inductors formed within circuits are usually capable of being tuned by physically altering the inductor to adjust its effective inductance. These tunable inductors are manufactured into a circuit that is then tuned to within desired operational limits by adjusting the effective inductance of such tunable inductors. There have been a number of basic ways of achieving this goal. Often a spiral-shaped inductor or a ladder-shaped inductor with multiple parallel shorts (also referred to herein as links or rungs) is utilized.
The ladder-shaped inductor is useful where fine tuning is required but space is not a premium consideration. Its inductance can be varied by severing (also referred to herein as breaching or trimming) one or more of the shorts across its vertical legs. The obtainable variance in inductance is substantially predictable and correlates to the number of, and spacing of, the ladder shorts.
Voltage-controlled oscillators used in communications circuits often make use of ladder-shaped inductors that can be adjusted in which a set of adjacent shorts can be severed in a single-ended fashion as part of a resonant circuit to overcome the impact of process variability on free-running oscillation frequency at a fixed tuning voltage. Such voltage-controlled oscillators may be utilized within automotive radar systems. The frequency can be lowered by severing one or more shorts in the ladder-shaped inductors to have a larger effective inductance. The increase of effective inductance by each severed short is however limited by the spacing between the shorts, which is constrained by the layout design rules. For example, a current tuning resolution for implementation of such ladder-shaped inductors on silicon is about 850 MHz-1000 MHz per short. There is therefore a desire for a more precise obtainable resolution of inductive values within such inductors so that a narrower oscillation frequency distribution of the circuits can be obtained, which in turn improves the yield during manufacturing of integrated circuits incorporating oscillators that implement such ladder-shaped inductors.